Solar cells having amorphous silicon as a light absorption layer (amorphous silicon solar cell) have been expected as s solar cell capable of lowering the cost, which have been already put to a practical stage partially. Japanese Published Unexamined Application Hei 1-309385 discloses an amorphous silicon solar cell using a flexible film as a substrate. The solar cell has an advantageous feature, compared with existent solar cells of using glass substrate, capable of providing optional curvature by the form of the film, capable of being carried about easily by its light weight, and capable of extending application ranges.
However, since amorphous silicon involves a problem that the performance is degraded when exposed to sunlight, and a single layer amorphous silicon solar cell capable of attaining a reduced cost has a conversion efficiency of only about 6% at the highest when it is formed on a film.
On the contrary, solar cells having a semiconductor layer of a chalcopyrite structure such as Cu(In,Ga)Se.sub.2 as a light absorption layer have been noted as the solar cells of high conversion efficiency, and research and development have been conducted vigorously for such solar cells. The layer constitution of the solar cell can include the structure, as shown in FIG. 1, in which a first electrode layer 2 comprising a thin film such as made of molybdenum, a p-type semiconductor layer 3 of a chalcopyrite structure, an n-type semiconductor layer 4 comprising CdS or the like and a second electrode layer comprising a thin film such as of ITO (indium/tin oxide) on a substrate 1.
Then, Solar Energy Materials and Solar Cells, 29, 1993) 163-173 discloses a solar cell having a CuInSe.sub.2 layer as a light absorption layer, and using a sheet of a simple noble metal such as molybdenum or titanium as a substrate. While the solar cell has a merit of being reduced in weight and having somewhat higher degree of freedom of the shape compared with the cell of using the glass substrate, it is difficult to be put into practical use since the cost for the sheet of the simple noble metal is high.
Solar Energy Materials and Solar Cells, 43 (1996) 93-98 discloses a solar cell having a CuInSe.sub.2 layer as a light absorption layer and using a polyimide film as a substrate. In this case, when the CuInSe.sub.2 layer is formed, it is necessary to apply a heat treatment in a hydrogen selenide atmosphere at a high temperature (400 to 600.degree. C.) for obtaining a film of good crystallinity. In the amorphous solar cell, such a heat treatment is not conducted. In the heat treatment described above, the polyimide film suffers from warp or degradation caused by hydrogen selenide. As a result, for example, break-down caused to the CuInSe.sub.2 layer, peeling of the CuInSe.sub.2 layer from the molybdenum electrode or peeling of the molybdenum electrode layer from the surface of the film. So the conversion efficiency of the solar cell is lowered.
Japanese Patent Laid-Open Publication No. Hei 5-259494 discloses a solar cell having a semiconductor layer of a chalcopyrite structure as a light absorption layer, and using a polyimide film, a polyethylene tetrafluoride film, or a copolymer film of ethylene tetrafluoride and propylene hexafluoride as a substrate. Further, it discloses a method of decreasing thermal injuries to the film by applying laser annealing to the semiconductor layer of the chalcopyrite structure in a state of keeping the substrate comprising the polymeric film described above at a temperature not higher than 0.degree. C.
However, since it is necessary to use a great amount of liquid nitrogen as a coolant for keeping the film at a temperature not higher than the 0.degree. C., it can not be said that the method is practical since the production cost is increased.
It is a subject of the present invention to provide a solar cell having a semiconductor layer of a chalcopyrite structure as a light absorption layer and using a flexible film as a substrate, having reduced weight, a high degree of freedom for the shape and capable of suppressing the production cost while keeping a high conversion efficiency, by preventing break-down or peeling caused to the semiconductor layer of the chalcopyrite structure owing to warp caused to the flexible film upon heat treatment or during use.